Wide-band gap II-VI compounds are expected to be one of the most vital materials for high-performance optoelectronics devices such as light-emitting diodes (LEDs) and laser diodes (LDs) operating in the blue or ultraviolet spectral range. Thin films were commonly grown using the conventional vapor-phase epitaxy (VPE) method. With the development of science and technology, new and higher requirements arose for material preparation. For this reason, novel epitaxial growth techniques were developed, including hot-wall epitaxy (HWE), metal organic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), metal organic molecular-beam epitaxy (MOMBE) and atomic layer epitaxy (ALE). Using these growth methods, film thickness can be controlled and quality can be improved. Examples of II-VI semiconductors are ZnS, ZnO, ZnSe, ZnTe, and CdTe.
Zinc oxide semiconductor materials comprising zinc and oxygen as constituent elements have attracted considerable attention since they can emit not only blue light but also near ultraviolet rays of 400 nanometers or less because of their wide band gap similarly to semiconductor materials such as gallium nitride and the like. Further, their applications to photo detector, piezoelectric device, transparent conductive electrode, active device and the like have also been expected without being limited to light emitting device. To form such a zinc oxide semiconductor material, various methods such as MBE method using ultra-high vacuum, sputtering, vacuum evaporation, sol-gel process, MO-CVD method, and the like have been conventionally examined. With respect to the light emitting device, the MBE method using ultra-high vacuum is widely used from the viewpoint of crystallinity.
Further, U.S. Pat. No. 4,278,913 describes a zinc oxide-based phosphor emits yellow light of high luminance under excitation of low-velocity electrons: xMIIF2.yMIIIF3.ZnO wherein MII is at least one divalent metal selected from the group consisting of beryllium, magnesium, calcium, strontium, barium, zinc and tin, MIII is at least one trivalent metal selected from the group consisting of aluminum, gallium, indium, thallium, yttrium and antimony, and x and y are numbers satisfying the conditions of 0.0001≦x+y≦0.1, 0≦x and 0<y. The zinc oxide-based phosphor is used as a fluorescent screen of a low-velocity electron excited fluorescent display device.